The present invention concerns frequency-conversion mixers and, more particularly, a novel linear mixer having reduced spurious responses.
It is known that frequency conversion of an input RF signal to a baseband output signal, by mixing the RF signal with a local oscillator (LO) waveform frequency, often generates spurious responses, and particularly those spurious responses related to multiples of the LO frequency. Such spurious responses are generally undesirable. For example, in an ultrasonic scanning system requiring an input bandwidth typically on the order of 1.2 MHz. to 6 MHz., the realized amplifier response is determined by the bandwidth of the integrated circuit amplifier stages utilized to obtain a required gain, and is on the order of 1-10 MHz. It is therefore possible for receiver thermal noise at the third harmonic of the signal frequency to be folded down to baseband and add with the desired signal, thusly degrading the output receiver signal-to-noise ratio. While some thermal noise protection may be provided by utilizing a bandpass filter between a wideband input amplifier and a subsequent demodulator, the output of the amplifier appears, under high gain conditions, as the sum of wideband thermal noise plus the desired signal. If a balanced demodulator, such as a Motorola MC1496 integrated circuit balanced demodulator, is used, the demodulator conversion efficiency will have decreased by only 10-14 dB. at the local oscillator third harmonic frequency, with respect to the conversion efficiency thereof at the fundamental frequency. If the desired signal has a 1MHz bandwidth, at a center frequency of about 2.5 MHz. (with an identical local oscillator frequency for conversion to baseband), the signal center frequency and local oscillator frequency third harmonics are at 6.75 MHz. and are well within the noise generation response range of the input wideband amplifier. Therefore, any noise at the third harmonic of the local oscillator frequency will also be translated down to baseband and reduce the system signal-to-noise ratio. Further, because a third-harmonic-rejecting bandpass filter must therefore be used such in such a receiver, if a multi-channel receiver is to be realized, that receiver must be rendered inoperative for some period of time while filters are changed when the system operating frequency is to be changed. It is therefore highly desirable that such a receiving system be able to operate at different frequencies without the necessity for making extensive changes and adjustments to the elements thereof.
While it is known that a third oscillator frequency harmonic problem can be eliminated by utilizing a sinusoidal LO waveform with a true multiplication circuit, such true-multiplication circuits are not currently available at required frequencies. Further, if the local oscillator waveform is digitally derived, that waveform is a two level waveform which would also generate third harmonic degradation of the signal-to-noise ratio even if a true multiplication circuit were to be used. Accordingly, a linear mixer driven by a local oscillator logic waveform and having an essentially zero magnitude response at at least the third harmonic of the local oscillator frequency is highly desirable.